The invention relates to a continuous casting plant having rollers or rolls with small diameters and great longitudinal extensions for supporting, guiding, bending straightening, and deforming a wide continuously cast strand, in particular a steel slab with a width of more than 1000 mm, wherein the length of the rollers or rolls exceeds the width of the strand (the width of the slab).
In continuous steel casting plants, in particular when slabs with a width of 1000 to 2500 mm or more are cast, a plurality of supporting rollers, arranged as close as possible to one another, must be provided below the water-cooled mould, in order to counteract a bulging of the strand skin caused by the ferrostatic pressure of the liquid core of the strand. Practice has shown that such deformations of the strand skin lead to cracks and thus to a deterioration of the quality of the cast product, particularly when rollers with relatively large diameters are used. The problem of supporting the strand skin becomes especially critical when a high casting speed (extraction rate of the strand) is necessary. Such so-called rapid casting plants with a casting speed of more than 1.2 m/min, for instance 1.6 to 2 m/min, require an especially careful construction and maintenance of the whole supporting and guiding roller-construction, since with increasing casting speed the demands for a precise strand support increase, and furthermore, the extraction force for the cast strand rises accordingly.
In modern continuous slab casting plants supporting roll constructions, such as the ones described in detail in German Auslegeschrift No. 1,458,158 and Austrian Pat. No. 290,750, have hitherto been used. With these known constructions, in order to keep the distances between the individual areas of support as small as possible, a plurality of rollers with small diameters is used for each area of support. The rollers are secured to a common, non-rotatable axis, one behind the other, in such a way that support members or axis holders, engage between the individual rollers. The support members or axis holders, in turn, are supported either on curved longitudinal carriers or on backing beams of a stationary frame construction. By supporting the stationary roller axis in a number of places, undue bending is prevented, and thus a much costlier construction is avoided, such as, for instance, the one described in U.S. Pat. No. 3,283,368. In this reference a solution to the problem of the bending of the thin and long supporting rollers is described in which, on the one hand, behind the stationarily mounted supporting rollers below the mould, further supporting rollers of wider diameters engage the thin rollers or that on the other hand the thin supporting rollers, are arranged at their ends in endless traveling chains so that the supporting rollers moving with the strand are, in turn, supported in or ride on a stationary path consisting of tracks arranged next to one another.
The following disadvantages are common to all these known supporting roller constructions, thus limiting their use in rapid slab casting plants:
a. By using a great number of roller bodies next to one another on a common axis, an extremely high number of bearings becomes necessary, and thus the disorder susceptibility of such a construction is correspondingly high, since supplying the bearing areas with lubricant is difficult and cannot be easily controlled.
b. Special cooling of the bearings -- in a closed cooling circuit -- is possible only at great expense due to the construction and -- in an open circuit -- the cooling agent can also reach the strand skin in undesired areas of the continuous casting machine.
c. The rollers, in particular rollers formed by roller bodies lying next to one another and having diameters of about 175 mm, for reasons of construction, cannot be provided with internal cooling, as has been suggested per se and has been known from German Pat. No. 1,908,763, but rather external cooling by means of spraying nozzles, being mainly provided for uniform cooling of the strand skin and adjusted accordingly, has to be employed. This has the grave disadvantage that, when the spraying nozzles are out of order -- for instance when they are clogged by impurities which may be contained in the cooling water due to insufficient maintenance of the water supply system -- there is the danger of overheating. For instance, in the case of such disorders in the external cooling system of the rollers, temperatures of about 700.degree.C were measured in the bearing elements, which at first caused the bearing grease to coke, then blocked the bearing, and finally deformed and bent the rollers. In extreme cases the extraction force may increase so enormously that the strand cracks and liquid steel flows out under very high pressure.
d. The roller bodies, arranged next to one another, cannot be driven for reasons of construction, and therefore the above mentioned friction resistance that increases with the casting speed (which has to be opposed by a greater extraction force) cannot be counteracted. Thus in all the known supporting roller constructions, the roller bodies are without a drive (see German Auslegeschrift No. 1,816,179, German Offenlegungsschrift No. 1,458,032 and U.S. Pat. No. 3,268,956). Since the solidifying strand does not shrink uniformly, it may happen that, for instance, lateral rollers on the edges of the strand no longer contact the surface of the strand and come to a stand-still. In this case they are heated up on one side very quickly and are destroyed. But, it may also happen that on the lower side of the strand, scales build up on the spraying nozzles and thus individual rollers become blocked and spans are scraped off the strand skin as it slides over the rollers.
e. Since the thin rollers cannot be driven, the extraction forces for the strand have to be applied by means of special driving stands whose rolls usually have diameters of about 500 mm. Since, especially in high capacity plants, the strand skin is very thin, it is desirable to apply the drawing forces to the strand skin as carefully as possible and to distributed them over the length of the strand. When a number of driving stands are arranged at distances between the non-driven supporting rollers of the above kind, there is the danger of an impermissible elongation of the strand skin and the formation of cracks in the transition areas, especially at increasing casting speeds. Therefore, it has hitherto been impossible to build a continuous casting machine, without providing for a highly complex construction, in which from the mould to the end of the area where the strand has a liquid core, only rollers with small diameters are used, wherein each individual roller can be used for extracting the strand, i.e. it can be driven.